The objective of this investigation is to establish the degree of diversity of lymphocyte subpopulations by using the natural property of some bacteria to bind to lymphocytes. We have shown both in lymphocyte suspensions and in blood smears that four human T cell subpopulations can be identified. In the mouse spleen three T cell subpopulations were also identified. The existance of two human B cell subpopulations in the peripheral blood and three mouse spleen B cell subpopulations was shown by bacterial adherence. Studies have also been done to determine the functions of the lymphocyte subpopulations by separating the cells on bacterial monolayers and stimulating them in vitro with mitogens of allogeneic cells. By using mutation and selection we showed that new strains of bacteria may be obtained to identify lymphocyte subpopulations. Such mutation experiments suggest that the mechanism of binding of bacteria to lymphocytes involves a "lectin" on the lymphocytes and mannose and/or glucose residues on bacterial cell walls. We propose to separate all lymphocyte subpopulations (4T and 2B) by sequential adherence to gelatin bound bacterial monolayers and to determine their function in vitro. Also, we shall attempt to isolate the materials on bacteria and on lymphocytes which can interact and to determine their molecular size and nature. We shall attempt to establish correlations between the size of the lymphocyte subpopulation and the peripheral blood smears and the increase or decrease in a particular function.